In implementing the technology of production of white plate glass-crystalline material from glasses of the known compositions, difficulties may be encountered related to both distortion of the band in the course of crystallization and its sagging between the conveying rolls of a roll table.
The known glass compositions for the fabrication of the white glass-crystalline material are given in the Table below.
Table __________________________________________________________________________ Com- posi- tion Redu- No. SiO.sub.2 Al.sub.2 O.sub.3 CaO MgO Na.sub.2 O MnO S.sup.2 F.sub.2 ZnO Zns Fe.sub.2 O.sub.3 cer __________________________________________________________________________ I 40- 5- 15- 45 2- 0.2- 0.2- 2 -- 2- -- -- 70 17 10 1 2 and or 5 less ZnO+S II 40- 5- 25- -- 0.5- -- -- 3.+-. 2.+-. -- -- -- 60 15 35 8 0.5 0.5 III 40- 5- 20- 38 3- -- -- 0 -- 1- -- 0-3 70 12 8 2 or 5 ZnO+S IV 40- 5-15 25- 0.1- 0.5- 0.1- 0.2- 0.5 0.5 -- 0.1 -- 63.5 15 35 3.5 8 1.5 0.8 6.5 3.5 1 __________________________________________________________________________
See Japanese patent No. 15508, (composition I), Inventor's Certificate of the USSR No. 201,608, (composition II), U.S. Pat. No. 3,170,780 (composition III) and Inventor's Certificate of the USSR No. 270,219 (composition IV).
The foregoing compositions are basically similar, their major components being CaO, Al.sub.2 O.sub.3, and SiO.sub.2. Present in all of the compositions is zinc sulfide or a combination of ZnO+S which perform a double function, acting as a crystallization catalyst and aiding in the production of the white-colored glass-crystalline material. The introduction of fluorides is needed for reducing the temperature at which crystallization is initiated.
In manufacturing glass-crystalline material great importance must be attributed to the viscosity characteristics of the glass employed as a base-material. The minimum viscosity value in crystallizing the above glasses amounts to 10.sup.8.0 poises, which is well below the conventional boundary of deformation, determined experimentally and corresponding to a viscosity of 10.sup.9.5 poises.
Such a low viscosity of the glass ensures crystallization of the pieces produced, which is likely to preserve their geometric configuration and linear parameters, provided only a continuous solid support is employed, but the low viscosity does not allow the production of the plate material by using a continuous rolling process. This is attributable to the fact that the viscosity values of the glasses subjected to crystallization during continuous rolling shall be in excess of 10.sup.9.5 poises, otherwise the glass band will be distorted due to sticking to the table rolls in the lehr or sagging between them.